This application is related to Japanese Patent Applications No. 2001-308901 filed on Oct. 4, 2001, and No. 2002-241850 filed on Aug. 22, 2002, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an ejector cycle system with two-step refrigerant decompression.
2. Description of Related Art
In an ejector cycle system described in JP-A-6-11197, an ejector sucks gas refrigerant evaporated in an evaporator at a low pressure side, and increases a pressure of refrigerant to be sucked into a compressor by converting an expansion energy to a pressure energy. In the ejector cycle system, a nozzle is constructed by a fixed throttle. Therefore, when heat load of the ejector cycle system changes, a flow amount of refrigerant flowing into the ejector is changed, and a decompression degree (throttle degree) in the nozzle is changed. Specifically, in a cooling device, when the outside air temperature is low and the heat load becomes smaller, the refrigerant amount evaporated in the evaporator is reduced, and a refrigerant circulating amount is reduced. In this case, a decompression degree of the refrigerant in the nozzle is excessively reduced, because a pressure loss generated in a fixed throttle is approximately proportion to the square of a flow rate. Accordingly, when the nozzle is selected to have a suitable decompression degree in the high heat load, a necessary throttle degree may be not obtained even when a discharge capacity of the compressor is made minimum, and the flow amount of refrigerant flowing into a radiator becomes larger more than a necessary amount. As a result, the flow rate in the radiator is excessively increased. As shown by the solid line (the actual state) in FIG. 6, refrigerant from the radiator is decompressed by the nozzle of the ejector without being sufficiently cooled in the radiator. Therefore, a cycle balance in the ejector cycle system is deteriorated, coefficient of performance (COP) of the ejector cycle system is deteriorated.
On the other hand, when the nozzle diameter is set smaller for obtaining a suitable decompression degree (throttle degree) in the low heat load, the load of the compressor is excessively increased when the refrigerant flow amount increases in the high heat load. Therefore, in this case, the COP of the ejector cycle system is decreased.
When carbon dioxide is used in the ejector cycle system, the high-pressure side refrigerant pressure becomes higher than the critical pressure of carbon dioxide in the high heat load, and the high-pressure side refrigerant pressure becomes lower than the critical pressure in the low heat load. FIG. 7 is a simulation graph showing a relationship between the outside air temperature and a suitable nozzle diameter where the COP becomes maximum when the ejector cycle system is used for an air conditioner. In FIG. 7, a diameter of a mixing portion of the ejector is 2.7 mm. As shown in FIG. 7, the suitable nozzle diameter in a super critical area is greatly different from the suitable nozzle diameter in a two-phase area (non-super critical area).
In view of the foregoing problems, it is an object of the present invention to provide an ejector cycle system having an ejector and a variable throttle unit which changes a decompression degree in accordance with high-pressure side refrigerant pressure.
It is another object of the present invention to provide an ejector cycle system which is operated while maintaining high COP.
According to the present invention, an ejector cycle system includes a compressor for sucking and compressing refrigerant, a radiator which cools refrigerant discharged from the compressor, an evaporator for evaporating the refrigerant, a gas-liquid separator for separating refrigerant from an ejector into gas refrigerant and liquid refrigerant. The ejector includes a nozzle for converting a pressure energy of high-pressure refrigerant from the radiator to a speed energy so that the high-pressure refrigerant is decompressed and expanded and gas refrigerant evaporated in the evaporator is sucked, and a pressure-increasing portion in which the speed energy is converted to the pressure energy so that the pressure of refrigerant is increased while refrigerant discharged from the nozzle and gas refrigerant from the evaporator are mixed. In the ejector cycle system, a decompression unit is disposed at an upstream side of the nozzle in a refrigerant flow direction for decompressing the refrigerant flowing from the radiator. Further, when pressure of the refrigerant before flowing into the decompression unit is equal to or higher than a predetermined pressure, a throttle open degree of the decompression unit becomes larger than that when the pressure of the refrigerant before flowing into the decompression unit is lower than the predetermined pressure. Accordingly, even when a nozzle radial dimension is suitably set in a pressure area higher than the predetermined pressure, it can prevent the throttle open degree is excessively decreased in a pressure area lower than the predetermined pressure, where heat load is small. Thus, in both cases of high-heat load and low-heat load, COP of the ejector cycle system can be effectively improved.
For example, when the pressure of refrigerant discharged from the compressor is increased to the critical pressure of the refrigerant, the predetermined pressure is set at the critical pressure of the refrigerant. Accordingly, in the super-critical area and in the two-phase area, the COP of the ejector cycle system can be effectively improved.
Preferably, a variable throttle unit is disposed between the evaporator and the gas-liquid separator, at a refrigerant inlet side, for controlling at least a flow amount of refrigerant flowing into the evaporator. Accordingly, even when the throttle open degree of the decompression unit is made larger, the capacity of the ejector cycle system can be accurately controlled by controlling the open degree of the variable throttle unit provided at a low-pressure side.
Preferably, the variable throttle unit is an expansion valve. In this case, a valve open degree of the variable throttle unit is controlled so that a refrigerant heating degree at an outlet side of the evaporator becomes a predetermined degree. Alternatively, the variable throttle unit is a pressure difference valve. In this case, a valve open degree of the pressure difference valve is controlled such that a pressure different between a refrigerant inlet side and a refrigerant outlet side of the pressure difference valve becomes a predetermined difference.